Electromagnetic actuators, such as linear or rotary solenoids, typically include a coil in electromagnetic communication with a movable armature. The coil is generally connected to a controllable driving circuit which varies the magnitude of current flowing in the coil and resultantly varies the strength of the magnetic field being produced by the coil. As the strength of the magnetic field is changed, the armature moves in response to the resulting change in the magnetic force being exerted on the armature.
Typically, the position of the armature is a function of both the magnitude of current flowing in the coil and the magnitude and direction of mechanical forces being exerted on the armature. The mechanical forces are exerted on the armature in response to the operating conditions of the system in which the electromagnetic actuator is operating. It is therefore advantageous to have a method of determining the position of the armature so that the operating conditions of the system can be indicated and used in connection with system diagnostics or a closed-loop control for the driving circuit.
The most common method of determining the position of the armature of an electromagnetic actuator is to connect an external sensor to the actuator. Such sensors often take the form of potentiometers or linear voltage differential transformers (LVDTs). While the addition of these sensors provides the desired information, they increase the cost and warehousing requirements of the actuator.
Attempts to provide position information without utilizing additional sensors have generally taken the form described in Japanese Patent Appl. No. 61-157418, published Jan. 20, 1988, and in Proceedings: 39th Relay Conference, Apr. 22-24, 1991, National Association of Relay Manufacturers, pp. 9-1 through 9-4. Both of the above publications disclose systems which determine the position of the armature by measuring the inductance of the coil in the actuator. Since inductance is a function of the air gap between the armature and the coil, the size of the air gap, and hence armature position, is determined by comparing measured inductance values to empirically determined inductance versus position characteristics. Systems of this type may provide inaccurate positional information for actuators that exhibit second-order characteristics since the electromagnetic flux varies, which in turn, effects the inductance values. Furthermore, as described in the Japanese Application, additional measurements and comparisons, e.g. coil temperature and magnetomotive force, are required to provide accurate indications of armature position.